How the torque calculation works
Torque is the moment of a force about an axis. Push perpendicular to a wrench handle and the twist you deliver is the force times the handle length:
T = F × r
where F is the force applied perpendicular to the arm and r is the perpendicular distance from the axis to the line of action. The SI unit is the newton-metre (N·m); US automotive practice uses pound-foot (lb·ft), and fastener specs often use pound-inch (lb·in). Conversions: 1 lb·ft = 1.35582 N·m and 1 lb·in = 0.112985 N·m.
Two practical corrections matter. First, if the force is not perpendicular to the arm, multiply by the sine of the angle between them (T = F·r·sin θ) — pulling along the wrench handle produces zero torque. Second, a torque wrench reading is affected by extensions: adding a crow's foot multiplies the effective arm length, so actual fastener torque runs higher than the dial reading.
Worked example
A 250 N push on a 0.3 m breaker bar gives T = 250 × 0.3 = 75 N·m — about 55.3 lb·ft. If the same job is done with a 0.45 m bar, the same arm strength delivers 112.5 N·m, which is why longer bars free stubborn bolts: torque scales linearly with arm length.
Where torque shows up
Fastener tightening (spec sheets list lb·ft or N·m), electric motors and engines (rated output torque), servo sizing for robotics, and bike or car drivetrains. Note the classic confusion: lb·ft (torque) is written ft·lb in some US manuals — same unit, same value, different word order.